Behavioral evidence suggests that many animals can detect that earth's magnetic field, however, very little is known about the neural basis of the detection of this sensory modality. In certain fish the electrical gradient is detected as they move through the magnetic field. However, for reasons to be given later in this application this mechanism is an unlikely one for other animals, particularly terrestrial ones. Another sensory mechanism appears to involve crystals of magnetite, for example in certain species of mud bacteria and honey bees. Recently, it has been shown that homing pigeons have magnetite-containing tissue located somewhere in the head (Walcott, Gould and Kirschvink, 1979). Since homing pigeons have been shown behaviorally to orient to the earth's magnetic field, it is important to pursue the hypothesis that this iron-containing tissue is involved in the detection of the earth's magnetic field. The specific aim of this proposal is to localize histologically iron-containing tissues in the pigeon's head. These tissues will be examined at the light and electron microscopic level to determine their structure and innervation. Surgical removal of the tissue(s) followed by behavioral tests will determine if in fact these tissues are involved in the sensory detection of magnetic fields. Subsequent neurophysiological recordings coupled with anatomical studies will elucidate the mechanism underlying, and the organization of, this sensory system.